Donor-π-acceptor based stable porphyrin sensitizers for dye-sensitized solar cells: Effect of π-conjugated spacers

Porphyrins are major sensitizers in dye-sensitized solar cells (DSSC) results in very high power conversion efficiency, however, aggregation tendency and visible range absorption avoids realistic applications. Thus, designing of novel porphyrins based sensitizers are essential to resolve the current existing issues. In this context, seven D–π–A porphyrin dyes (LG1–LG7) engineered with 3-ethynyl phenothiazine tethered at the meso-position and π-spacers such as 4-ethynyl phenyl (LG1), 5-ethynylthiophene (LG2), 5-ethynyl furan (LG3), 2,1,3-benzothiadiazole (BTD)–phenyl (LG6) and 2,1,3-benzothiadiazole (BTD)–thiophene (LG7) were incorporated between porphyrin macrocycle and anchoring carboxylic acid. Similarly, π-spacers 4-ethynyl phenyl (LG4) and 4-ethynylthiophene (LG5) were functionalized between porphyrin and anchoring cyanoacrylic acid. LG5 and LG6 showed significant near infrared absorption resulted highest efficiency of 10.20% and 9.64% among other derivatives. UV-vis-NIR absorption, cyclic voltammetry and density functional theory calculations of LG1-LG7 suggested that LG5 exhibits strong absorption and optimized lowest unoccupied molecular orbital (LUMO) aid to inject electrons very effectively from exited state of dye into TiO2 conduction band. Current density-voltage (J-V) of LG1-LG7 revealed that LG5 exhibits highest short-circuit current density of 21.01 mAcm-2 resulting the power conversion efficiency of 10.20% in a liquid I-/I3- redox couple electrolyte. Panchromatic IPCE response of LG5 observed in between 400-900 nm, when compared to other derivatives. Thus, these results suggest that LG5 attained highest efficiency in liquid electrolyte based DSSC. Subsequently, durability studies of LG5 performed by continuous light exposure have shown that this sensitizer retained 80% initial efficiency after 1000 h. Therefore, effect of spacer length and anchoring significantly contributed to improve the efficiency in liquid electrolyte, which is very useful to make the efficient future generated dye sensitized solar cells